Asphalt paving material is comprised of an asphaltic binder and aggregates of various particle sizes, including both coarse and fine aggregate materials. Because the equipment needed to produce asphalt paving material is expensive and the space required extensive, asphalt paving material is typically produced in a production facility that is dedicated to such purpose. Consequently, it is frequently necessary to transport the asphalt paving material from its place of origin to an asphalt paving machine at a remote paving site. The asphalt paving material is usually transported in dump trucks to an asphalt paving machine or to a material transfer vehicle that completes the transfer to the asphalt paving machine.
An asphalt paving machine is a self-propelled vehicle that is driven by a wheeled or tracked drive system. In a common type of paving machine, an asphalt receiving hopper is located at the front end of the machine to receive asphalt paving material, and a slat-type hopper conveyor located below the asphalt receiving hopper transfers the asphalt paving material from the hopper to a distribution assembly comprising a transverse distributing auger that is mounted at the rear of the machine. The asphalt paving material is deposited onto and across the roadway or other surface to be paved by the distributing auger, and a floating screed located behind the distributing auger compacts the asphalt paving material to form an asphalt mat.
Sometimes, asphalt paving material is discharged directly from a delivery truck into the asphalt receiving hopper of the asphalt paving machine. A dump-type delivery truck is unloaded by raising the truck bed and allowing the asphalt paving material to slide down the bed into the receiving hopper. When the truck bed is raised, it should not come into contact with the receiving hopper and should not be carried by or ride on any portion of the paving machine. For smaller-capacity dump trucks, contact with the paver is not often a problem. However, such contact can be a problem when large tractor-semitrailer units are used as delivery vehicles, particularly when the truck bed is extended to its highest point. In addition, if a delivery truck contacts the paver so that a portion of the weight of the delivery truck is carried by the paver as the paving operation is carried out, the screed elevation may be changed, which will affect the smoothness of the finished asphalt mat.
When asphalt paving material is delivered to the paving machine by delivery trucks, it is frequently necessary for a series of delivery trucks to move into contact with the front end of the paving machine to serially discharge their loads into the paving machine's asphalt receiving hopper. This method of delivery requires multiple truck maneuvers that are often difficult to achieve without stopping the paving machine. However, when a paving machine stops and subsequently restarts, its floating screed will produce a dip in the asphalt mat (when the machine stops) and a bump (when it restarts). In addition, because a typical delivery truck carries more asphalt paving material than can be unloaded quickly into the receiving hopper of the asphalt paving machine, it is frequently necessary for the paving machine to push the truck in the paving direction as the truck is unloaded while paving proceeds. This may be difficult to manage when the paving machine is proceeding through intersections or operating on curved sections of the roadway. Furthermore, the ability of the paving machine operator to place a smooth mat on the roadway will be affected by the rate of feed of asphalt paving material to the paving machine. This requires planning for proper scheduling of delivery trucks and coordination with the asphalt production facility. However delays at the production facility or traffic encountered by the delivery trucks can thwart the efforts of the most careful planners. Consequently, because it is desirable to keep the paving machine moving at all times during an asphalt paving operation, and since delivery trucks must be unloaded as they are pushed along in the paving direction, it is frequently necessary to have delivery trucks queue up near the paving machine to ensure that a loaded truck is available to move quickly into unloading position as an unloaded truck is moved out of the way. This may result in heat losses in the asphalt paving material in the waiting trucks, which can affect the quality of the asphalt mat being created by the paving machine.
A delivery truck can also be used to deliver the asphalt paving material to a windrow on the roadway in front of the paver. If the delivery truck is a dump truck, the windrow is usually formed by a spreader box or a windrow blending unit. If a spreader box is used, it will be pulled behind the truck, and the truck bed raised to deposit the asphalt paving material into the box. As the truck moves forward, the asphalt paving material is uniformly metered out of the box onto the roadway. If a windrow blending unit is used, it will typically be attached to a small front-end loader, and the asphalt paving material dumped onto the existing roadway across the width of the truck bed. The windrow blending unit will fold the asphalt paving material into a windrow as the blending unit is pushed forward by the loader. A bottom-unloading truck may also be used to deposit asphalt paving material in the form of a windrow onto the roadway.
When asphalt paving material is deposited on the roadway in the form of a windrow, it may be picked up from the roadway surface by a windrow elevator that is attached to the front of the asphalt paving machine. However, because it is desirable to keep the paving machine moving at all times while the paving machine is being operated, the windrow method of delivery may still require delivery trucks queueing up near the paving machine to insure that a windrow of asphalt paving material is available as soon as it is needed by the paving machine. This may also result in heat losses in the asphalt paving material in the waiting windrows, which can affect the quality of the asphalt mat being created by the paving machine.
For all of the difficulties associated with the timely delivery of asphalt paving material by individual delivery trucks to a paving machine or in the form of windrows, material transfer vehicles have been used in recent years to transport asphalt paving material to an asphalt paving machine. One type of material transfer vehicle may be used to shuttle asphalt paving material between the delivery trucks or windrows and the asphalt paving machine. Yet another type of material transfer vehicle is adapted to move alongside the asphalt paving machine while being tethered to a delivery truck to allow for the transfer of asphalt paving material from a delivery truck to an asphalt paving machine without requiring either the delivery truck or the material transfer vehicle to make direct contact with the asphalt paving machine, thus reducing the time required for delivery truck maneuvers and reducing the risk that the paving machine will have to stop during the paving operation. Still other material transfer vehicles are equipped with windrow pick-up heads that can pick up a windrow of asphalt paving material that has been dumped on the roadway as the material transfer vehicle is moved into the windrow. These vehicles are adapted to move alongside an asphalt paving machine and transfer the asphalt paving material from the windrows into the receiving hopper of the asphalt paving machine.
Some material transfer vehicles, such as the Roadtec MTV-1100, the Roadtec SB-1500 and the Roadtec SB-2500, are self-propelled. Self-propelled material transfer vehicles may include a large-capacity truck-receiving hopper or a window pick-up head, and an inclined loading conveyor extending upwardly from the hopper or pick-up head. In self-propelled material transfer vehicles equipped with a truck-receiving hopper, a transversely oriented auger may be provided in the truck-receiving hopper to urge asphalt paving material onto the loading conveyor. The asphalt paving material is carried upwardly by the loading conveyor from the truck-receiving hopper or pick-up head and discharged off the elevated output end of the loading conveyor into a chute mounted on the lower end of a discharge conveyor, or into an intermediate surge bin that is sized to hold the entire load of a delivery truck. The discharge of asphalt paving material off the elevated output end of the loading conveyor so that it may fall under the influence of gravity into a chute or surge bin assists in preventing undesirable segregation of the various particulate components of the asphalt paving material by particle size.
Material transfer vehicles of the type that are equipped with a surge bin typically include an auger and a conveyor in the surge bin that are adapted to transfer the asphalt paving material to the discharge conveyor. The auger in the surge bin re-blends the asphalt paving material to eliminate particle size segregation that occurs as the material is transported. It also helps to keep the heat in the asphalt paving material evenly distributed throughout the entire load of material obtained from the delivery truck.
Discharge conveyors that are mounted on self-propelled material transfer vehicles with and without surge bins are generally pivotable about an essentially vertical axis so that the transfer vehicle can be positioned alongside an asphalt paving machine that is laying an asphalt mat and rapidly discharge asphalt paving material into the hopper of the paving machine as the material transfer vehicle moves with the paving machine along the roadway. Furthermore, these discharge conveyors are configured so that their discharge end may be raised and lowered to position the discharge outlet advantageously with respect to the asphalt receiving hopper of a paving machine. The moveable nature of these discharge conveyors allows for some flexibility in locating the material transfer vehicle adjacent to the asphalt paving machine. Because of its rapid loading and unloading capabilities, a self-propelled material transfer vehicle equipped with a surge bin can rapidly shuttle between delivery trucks or windrows at a pick-up point and an asphalt paving machine that is laying an asphalt mat at a paving site so that there is less likelihood that the paving machine will have to stop paving because of a lack of asphalt paving material.
Other types of material transfer vehicles, such as the Cedarapids MS2 and the Cedarapids MS5, are not self-propelled, but are adapted to be tethered to an asphalt paving machine. These machines may have windrow pick-up heads that are adapted to pick up a windrow of asphalt paving material that has been dumped on the roadway as the machine is moved into the windrow. These machines also include a loading conveyor that that is attached to the windrow pick-up head to carry the asphalt paving material up to a chute that is positioned over the asphalt receiving hopper of the asphalt paving machine.
Some of the self-propelled material transfer vehicles, such as the Roadtec SB-1500 and the Roadtec SB-2500, can alternatively be provided with either a truck-receiving hopper or a windrow pick-up head. However, these machines cannot be converted between a configuration that is equipped with a truck-receiving hopper and a configuration that is equipped with a windrow pick-up head.
When a self-propelled material transfer vehicle is used in an asphalt paving process, a delivery truck can be stopped a significant distance away from the paving machine in order to unload its load from a stopped position into the material transfer vehicle or onto the roadway. Thus, the delivery truck can unload faster when not unloading directly into the paving machine, and the unloading location may be selected so that there is no danger of the raised dump bed of the truck hitting power lines or tree limbs on the side of the roadway. The use of a material transfer vehicle also reduces the need for delivery trucks to queue up at the paving machine in order to keep it moving, thereby cutting the waiting time of the delivery trucks and thereby reducing truck operating costs.
Even though material transfer vehicles are quite versatile, it would be desirable if a material transfer vehicle could be easily adapted to receive asphalt paving material in a truck-receiving hopper or from a windrow pick-up head.